GB2258107A

GB2258107A - Operational amplifiers and response speeds thereof

Info

Publication number: GB2258107A
Application number: GB9122286A
Authority: GB
Inventors: Bang-Won Lee; Kyoung-Un Hwang
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 1991-07-26
Filing date: 1991-10-21
Publication date: 1993-01-27
Anticipated expiration: 2011-10-21
Also published as: JPH0738538B2; KR940001816B1; CN1069152A; TW209323B; GB9122286D0; CN1028824C; US5223753A; JPH0555837A; KR930003522A; GB2258107B

Abstract

In an operational amplifier or a comparator, a slew rate speed-up circuit improving the slew rate, without extra power consumption comprises a first pull-up transistor 9 having a gate connected to an output terminal 23 of a differential amplifier 1, 2 and 3, and having a channel connected between a power supply voltage terminal VDD and a first output terminal 25, a second pull-up transistor 10 having a gate connected to the output terminal 23 of the differential amplifier, a second output terminal 26 connected to one end of the channel of the second pull-up transistor 10, a current regulating means 30, connected between the first output terminal 25 and the second output terminal 26, having one common current path 14, one pull-up current path 12 and one pull-down current path 13, a first pulldown transistor 8 having a channel connected between the first output terminal 25 and a ground or reference voltage terminal VSS, and a second pull-down transistor 8 having a channel connected between the second output terminal 26 and the ground or reference voltage terminal VSS. The amplifier can be used in a semiconductor memory. <IMAGE>

Description

223S1:37 OPERATIONAL AMPLIFIERS AND RESPONSE SPEEDS THEREOF The present

invention relates to operational amplifiers or comparators 5 and is concerned particularly with the response speeds thereof.

Many electronic circuits widely use the operational amplifier or the comparator, which produces an output signal by summing, subtracting or differential amplifying an input signal of an inverting terminal and a non- inverting terminal. In such an operational amplifier, the degree of response of an output signal as a function of an input signal may have an important effect on the performance and reliability of the electronic circuit. Especially, in a comparator used for a semiconductor memory device, etc., if the response speed is slow, a malfunction of the memory device operating at high speed, as well as a slow data access speed, can be caused.

Figure 1 of the accompanying diagrammatic drawings is a circuit diagram illustrating a known comparator (or an operational amplifier) of the prior art. As shown in Figure 1, it will be appreciated that a comparator and a differential amplifier of N-channel input type are the same in construction. Therefore, when the potential of a first input terminal 21 of comparator 20 is higher than that of a second input terminal 22, P type MOS transistors 4 and 5 and N type MOS transistors 7 and 11 are turned on, thereby to drop the potential of a second output terminal 24. On the contrary, if the potential of the second input terminal 22 is higher than that of the first input terminal 21, a power supply voltage VDD is charged on the second output terminal 24 through a P type MOS transistor 10. In this case, the voltage gain Avol of first output terminal 23 and the voltage gain Avo, of the second output terminal 24 are calculated respectively as follows:

A Vol =9M2/9M6 AV02= 19mi (SIO/S6)1/(gdslo + gds,,) wherein gmj, gm. and 9M6 are the transconductance of N type MOS transistors 1 and 2 and P type MOS transistor 6, respectively, gds the channel conductance, and S the ratio of the width of channel to the length thereof.

In the case that the output voltageVOUT iSpositively increased, the slew rate SR at the second output terminal 24 has a maximum value when the pulldown current Ill flowing into a ground voltage terminal Vss from the second output terminal 24, through the N type MOS transistor 11, is "0". In the case that the output voltage is negatively increased, the slew rate SR at the second output terminal 24 has a maximum value when the pull-up current 110 flowing into the second output terminal 24 from the power supply voltage terminal VDD, through the P type MOS transistor 10, is %1011 However, at the second output terminal 24, there exists the load capacitance CL of a parasitic capacitor 15, which makes the load current iL flow. The load capacitance CL and the load current il. have an important effect on the slew rate SR when the potential of the second output terminal 24 is changed from logic "LOW" to logic MIGE' or from logic MIGS' to logic "LOW". In detail, the slew rate SR is defined as the differential coefficient of the output voltageVOUTwith respect to time, and on the other hand the load current iL indicates the value obtained by subtracting the pull-down current 11, from the pull-up current I10. Thus, the slew rate SR can be expressed as follows.

SR dV ,,,jdt iLICL = 1 (110-Ill)ICL 1 As shown in the above expression concerning the slew rate SR, in order to increase the slew rate SR (or to have a fast response speed of an output signal as a function of an input signal), the load capacitance CL should be decreased or the load current iL should be increased. The value of the load capacitance can not be changed because the load capacitance, as a parasitic element, has an inevitably fixed value in a given circuit construction, but it is possible to increase the load current iL by increasing the size of P type MOS transistor 10 and N type MOS transistor 11. However, increase in the size of MOS transistors 10 and 11 at the output results in an undesirable increase of power consumption of the circuit.

I Preferred embodiments of the invention aim to provide a circuit capable of improving a slew rate without increasing power consumption of an output terminal in a comparator (or operational amplifier).

According to a first aspect of the present invention, there is provided a circuit for improving response speed of an output signal with respect to an input signal of an operational amplifier having a differential amplifier, said circuit comprising:

a first pull-up transistor with a gate connected to an output terminal of said differential amplifier; a first output terminal connected to one terminal of a channel of said first pull-up transistor; a first pull-down transistor having a channel connected between said first output terminal and a ground voltage terminal; a second pull-up transistor having a gate connected to said output terminal of said differential amplifier and having one terminal of a channel thereof connected to a power supply voltage terminal; a second pull-down transistor connected between said ground voltage terminal and said second pull-up transistor; a second output terminal connected to the other terminal of said channel of said second pull-up transistor and said pull-down transistor; and current regulating means connected between said first output terminal and said second output terminal, and having common current path means, pull-up current path means and pull-down current path means.

Preferably, a current direction of said common current path means is determined in dependence on a potential difference between said first output terminal and said second output terminal.

Preferably, said pull-up current path means comprises a channel of a first metal-oxide-semiconductor"transistor having a gate connected to said first output terminal, said first metal-oxide-semiconductor transistor being connected between said power supply voltage terminal and said second output terminal.

Preferably, said pull-down current path means comprises a channel of a second metal-oxide-semiconductor transistor having a gate connected to said -5 first output terminal, said second metal-oxide-semiconductor transistor being connected between a ground voltage terminal and said second output terminal.

Preferably, said common current path means comprises of a resistor connected between said first output terminal and said second output terminal.

Preferably, the conductive type of said first and second pull-up transistors and said second metal-oxide-semiconductor transistor is the opposite of that of said first and second pull-down transistors and said first metal-oxide-semiconductor transistor.

According to another aspect of the present invention, there is provided a circuit for improving response speed of an output signal with respect to a given input signal of an operational amplifier having a differential amplifier, said circuit comprising:

a first output terminal; a second output terminal; a first pull-up transistor having a gate coupled to an output terminal of said differential amplifier and having a channel connected between a power supply voltage terminal and said first output terminal; a second pull-up transistor having a gate coupled to said output terminal of said differential amplifier and having a channel connected between said power supply voltage terminal and said second output terminal; a first pull-down transistor having a channel connected between said 30 first output terminal and a ground voltage terminal; a second pull-down transistor having a channel connected between said second output terminal and said ground voltage terminal; a third pull-up transistor having a gate coupled to said first output terminal and having a channel connected between said power supply voltage terminal and said second output terminal; a third pull-down transistor having a gate coupled to said first output terminal and having a channel connected between said ground voltage terminal and said second output terminal; and resistive means connected between said first output terminal and said second output terminal.

Preferably, the conductive type of said first and second pull-up transistors and said third pull-down transistor is the opposite of that of said first and second pull-down transistors and said third pull-up transistor.

The invention extends to an operational amplifier provided with a circuit according to any of the preceding aspects of the invention, and to a semiconductor memory device provided with such a circuit or operational amplifier.

In accordance with another aspect of the present invention, an operational amplifier, having one differential amplifier and a first pull- up transistor with a gate connected to the output terminal of the differential amplifier, includes a first output teiminal, a second pullup transistor with a gate connected to the output terminal of the differential amplifier and with channel connected between a power supply voltage terminal and the first output terminal, a second output terminal connected to either one terminal of the channel of the first pull-up transistor, a current regulating means connected between the first output terminal and the second output terminal and having one common current path, one pull-up current path and one pull-down current path, a first pull-down transistor with a channel connected between the second output terminal and a ground voltage terminal, and a second pull- down transistor with a channel connected between the first output terminal and a ground voltage terminal.

For a better understanding of the invention, and to show how the same 10 may be carried into effect, reference will now be made, by way of example, to Figures 2 and 3 of the accompanying diagrammatic drawings, in which:

Figure 2 is a circuit diagram illustrating one example of a preferred embodiment of a comparator (or an operational amplifier) according to the present invention; and Figure 3 is a view illustrating one example of an operational waveform of the comparator of Figure 2.

In Figures 1 and 2, like parts are designated by like reference numerals. In Figure 2, a gate of a P type MOS transistor 9 is connected to an output terminal 23 of a differential amplifier, and a channel thereof is connected between a power supply terminal VDD and a first output terminal 25. An N type MOS transistor 8 has a gate connected to a gate of an N type MOS transistor 7, and a channel connected between the first output terminal and a ground voltage terminal Vss. A current regulating circuit 30, comprised of an N type MOS transistor 12, a P type MOS transistor 13 and a resistor 14, is connected between the first output terminal 25 and a second output terminal 26, the second output terminal 26 corresponding to the second output terminal 24 of Figure 1. The gate of the N type MOS transistor 12 is connected to the first output terminal 25, and a channel thereof is connected between the power supply voltage VDD and the second output terminal 26. A gate of the P type MOS transistor 13 is connected to the first output terminal 25, and a channel thereof is connected between the second output terminal 26 and the ground voltage terminal Vss. The resistor 14 connects the first output terminal 25 to the second output terminal 26. In the circuit as shown in Figure 2, it should be noted that the first and second P type MOS transistors 9 and for use in pull-up are equal in size, and the first and second N type MOS transistors 8 and 11 for use in pull-down are also equal in size.

In Figure 3, an input waveform 3 1, a waveform 32 at the first output terminal 25 and a waveform 33 at the second output terminal 26 are shown.

It is assumed that an input signal, referred to as "a first input signaP, of logic low i.e., "0" level is applied to the first input terminal 21, and a signal, referred to as "a second input signaP, such as the input waveform 31 of Figure 3 is applied to the second input terminal 22. Since the potential of the second input signal is higher than that of the first input signal, the potential of the output terminal 23 of the differential amplifier goes to logic low.

Thereby, the P type MOS transistors 9 and 10 for use in pull-up are turned on, thus charging the first output terminal 25 and the second output terminal 26 up to the power supply voltage VDD level. In this case, if there were no capacitor 15 at the second output terminal 26, the voltage of the first output terminal 25 and the second output terminal 26 would be identical to each other and then current would not flow between the first output terminal 25 and the second output terminal 26. However, as described above, it is unavoidable that the load capacitance CL caused by the parasitic capacitor 15 at the second output terminal 26 exists. Therefore, at the first output terminal 25, the voltage such as the waveform 32 of Figure 3 is formed, and at the second output terminal 26, due to the load capacitance CL, the voltage such as the waveform 33 of Figure 3 is formed. The waveform 33 of the second output terminal 26 has a more gentle gradient, i.e. a more gentle slew rate than that of the waveform 32 of the first output terminal 25. This is because of the charging time of the load capacitance CL. That is, the potential of the first output terminal 25 is higher than that of the second output terminal 26 by as much as AV at time tl. If the potential difference AV becomes greater than the threshold voltage of the N type MOS transistor 12, the N type MOS transistor 12 is turned on and then current IX flows into the second output terminal 26 until the potential of the first and second output terminals 25 and 26 become identical. Furthermore, the current 'R5 caused by the potential difference between the first output terminal 25 and the second output terminal 26 at the resistor 14, flows into the second output terminal 26. That is, the amount of the current flowing into the second output terminal 26 becomes (I10-Ill)+'X+'R. This amount of current flowing into the second output terminal 26 is increased by as much as (i,+i R), in comparison with that of Figure 1. As a result of the increase of output current, the charging time of the capacitor 15 becomes shorter accordingly, thereby improving the slew rate.

In the same way, if the potential of the second input signal is lower than that of the first input signal which is logic "0" level, the current flowing into the ground voltage terminal Vss becomes the sum of the current iy flowing through the P type MOS transistor 13, the current iy flowing into the first output terminal 25 through the resistor 14, the direction thereof being opposite to the direction of the above current iR, and the pull-down current ill. The current direction flowing into the first output terminal 25 through the resistor 14 is determined in dependence on the potential difference between the first output terminal 25 and said second output terminal 26. Accordingly, the discharging time of the load capacitance CL becomes shorter, thus improving the slew rate.

Consequently, since the waveform 33 of the second output terminal 26 is shifted in the direction of the arrows in Figure 3, the response speed (or the slew rate) of an output signal as a function of the input waveform 31 becomes faster.

As described above, in an operational amplifier or a comparator, the slew rate of the output signal of a circuit can be improved without increasing power consumption in stand-by.

A preferred embodiment of the present invention has been particularly shown and described with reference to employing a comparator of an Nchannel input type. However, even if a comparator of a P-channel input type is used, or even if a signal such as an input waveform 31 of Figure 3 is used as a first input signal with setting a second input signal of logic T', level, it will be understood by those skilled in art that the operation and aim achieved by the described preferred embodiment can equally be performed.

In this specification, reference is made conveniently and conventionally to a "ground voltage". It will be understood by those skilled in the art, however, that such a term includes within its scope reference potentials other than zero volts.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A circuit for improving response speed of an output signal with respect to an input signal of an operational amplifier having a differential amplifier, said circuit comprising:

a first pull-up transistor with a gate connected to an output terminal of said differential amplifier; a first output terminal connected to one terminal of a channel of said first pull-up transistor; a first pull-down transistor having a channel connected between said first output terminal and a ground voltage terminal; is a second pull-up transistor having a gate connected to said output terminal of said differential amplifier and having one terminal of a channel thereof connected to a power supply voltage terminal; a second pull-down transistor connected between said ground voltage terminal and said second pull-up transistor; a second output terminal connected to the other terminal of said channel of said second pull-up transistor and said pull-down transistor; and current regulating means connected between said first output terminal and said second output terminal, and having common current path means, pull- up current path means and pull-down current path means.

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2. A circuit as claimed in claim 1, wherein a current direction of said common current path means is determined in dependence on a potential difference between said first output terminal and said second output terminal.

3. A circuit as claimed in claim 1 or 2, wherein said pull-up current path means comprises a channel of a first metal-oxidesemiconductor transistor having a gate connected to said first output terminal, said first metal-oxidesemiconductor transistor being connected between said power supply voltage terminal and said second output terminal.

4. A circuit as claimed in claim 1, 2 or 3, wherein said pull-down current path means comprises a channel of a second metal-oxide- semiconductor transistor having a gate connected to said first output terminal, said second metal-oxide-semiconductor transistor being connected between a ground 15 voltage terminal and said second output terminal.

5. A circuit as claimed in claim 1, 2, 3 or 4 wherein said common current path means comprises of a resistor connected between said first output terminal and said second output terminal.

6. A circuit as claimed in claims 3 and 4 or 3 to 5, wherein the conductive type of said first and second pull-up transistors and said second metal-oxidesemiconductor transistor is the opposite of that of said first and second pulldown transistors and said first metal-oxidesemiconductor transistor.

7. A circuit for improving response speed of an output signal with respect to a given input signal of an operational amplifier having a differential amplifier, said circuit comprising:

a first output terminal; a second output terminal; a first pull-up transistor having a gate coupled to an output terminal of said differential amplifier and having a channel connected between a power supply voltage terminal and said first output terminal; a second pull-up transistor having a gate coupled to said output terminal of said differential amplifier and having a channel connected between said power supply voltage terminal and said second output terminal; a first pull-down transistor having a channel connected between said first output terminal and a ground voltage terminal; a second pull-down transistor having a channel connected between said second output terminal and said ground voltage terminal; a third pull-up transistor having a gate coupled to said first output terminal and having a channel connected between said power supply voltage terminal and said second output terminal; a third pull-down transistor having a gate coupled to said first output terminal and having a channel connected between said ground voltage terminal and said second output terminal; and resistive means connected between said first output terminal and said second output terminal.

8. A circuit as claimed in claim 7, wherein the conductive type of said first and second pull-up transistors and said third pull-down transistor is the opposite of that of said first and second pull-down transistors and said third pull-up transistor.

9. A circuit for improving response speed of an operational amplifier, the 5 circuit being substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.

10. An operational amplifier provided with a circuit according to any of the preceding claims.

11. A semiconductor memory device provided with a circuit or operational amplifier according to any of the preceding claims.